Perhaps predictibly, someone showed up in the comments to my post on Medicare and Social Security to argue that liberal analysts have very serious plans to cut Medicare's costs, which is why we need universal coverage, so that we can implement those very serious plans.
I hear this argument quite often, and it's gibberish in a prom dress. Any cost savings you want to wring out of Medicare can be wrung out of Medicare right now: the program is large and powerful enough, and costly enough, that they are worth doing without adding a single new person to the mix. Conversely, if there is some political or institutional barrier which is preventing you from controlling Medicare cost inflation, than that barrier probably is not going away merely because the program covers more people. Indeed, to the extent that seniors themselves are the people blocking change (as they often are), adding more users makes it harder, not easier, to get things done.